The present invention relates to optical metrology, and, in particular, to an apparatus and methods for increasing the accuracy of measurement of distance to a reflecting surface.
Noise levels in optical systems, whether directed toward imaging or measurement of an illuminated object or scene, are detrimentally affected by speckle and by the compound nature of specular and diffuse reflection at a reflecting surface. Speckle arises because of interference effects due to the coherence of the illuminating light.
Light may reach a given detector element via multiple paths. If the light rays reaching the detector from two points on the surface of the illuminated scene bear a defined phase relationship with respect to one another, the detected intensity will be a function of that phase, increasing when the rays are in phase, and decreasing when the rays are out of phase. While useful in certain applications, speckling generally degrades imaging, adding a noise component which may approach the level of the signal itself.
Similarly, if a surface is illuminated by a substantially collimated beam, light that is diffusely scattered from a surface rather than specularly reflected will generally impinge upon the detector plane at angles other than those corresponding to the illuminating beam. One component of the diffuse reflection, however, referred to herein as the xe2x80x9cdiffuse lobe,xe2x80x9d is close in direction to that of the specular reflection and similarly close in polarization to specular reflection from a reflecting surface. However, neither the direction nor the polarization are identical to that of specular reflection. In certain metrological applications, it is the diffuse lobe that is of interest since it has characteristics of light emanating from a surface rather than the characteristics of the source of illumination. Specular reflection from a metal surface thus similarly degrades imaging and metrological resolution.
In order to despeckle the illumination, it is useful to decohere any coherent light source. Various methods are known, including the use of a fiber optic multichannel array, as described in U.S. Pat. No. 5,029,975. A survey of known speckle reduction techniques is provided by T. S. McKechnie, xe2x80x9cSpeckle Reduction,xe2x80x9d in J. C. Dainty, ed., Laser Speckle and Related Phenomena, Springer, Berlin, 1975, pp. 123-170, which is incorporated herein by reference.
One example of a metrological application that may be adversely affected by speckle is that of conoscopy, a form of holography that may be practiced using incoherent light. In ordinary holography, coherent light emanating from a source region is caused to interfere with a coherent reference beam in order to construct an interferogram in which the characteristics of the source region in two or three dimensions are encoded. Conoscopy is a distinct interferometric technique capable of determining the distance to one or more points within an object volume without employing a reference beam. Instead, light emanating from a source region is prepared in a defined state of polarization and then passed through an anisotropic optical element in which one polarization suffers phase retardation with respect to the other. The two polarization components emerging from the anisotropic optical element interfere with one another, producing a interferogram in the detector plane. Conoscopy is the subject of various patents, including U.S. Pat. Nos. 4,602,844, 4,976,504, 5,081,540, and 5,081,541, which patents are incorporated herein by reference. The source region may be illuminated with a grid (or xe2x80x9ccloudxe2x80x9d) of points, or with a line. However interference effects attributable to the coherence of the source are detrimental to the sensitivity of the measurement technique. Conoscopic resolution is typically degraded if the light emanating from a source region is the product of both specular and diffuse reflection of the illuminating light.
Accordingly, for this and other metrological techniques, it is desirable to overcome speckle and to distinguish, in detection, between diffuse and specular components of light reflected by an illuminated surface.
In accordance with a preferred embodiment of the present invention, there is provided a method for enhancing the proportion of diffusely scattered light to light specularly reflected from a surface. The method has the steps of receiving a beam of light having a direction of predominant polarization, directing the beam through a wedge module so as to illuminate the scene, and detecting light reflected from the scene. The wedge module has a first optically anisotropic wedge, a second optically anisotropic wedge inclined relative to the first optically anisotropic wedge, and an optical compensation plate having an interface parallel to a face of the first optically anisotropic wedge and a face of the second optically anisotropic wedge so as to compensate any change in optical path of the beam of light. The polarization axes of the first and second wedges are substantially parallel to the direction of predominant polarization of the illuminating beam whereas the light emanating from the illuminated surface in a perpendicularly polarized direction is detected.